In an electronic device such as a semiconductor device, a porous film may be used. As the porous film, for example, a film formed of a low dielectric constant material such as a SiOC film is used. In manufacturing of such an electronic device, a resist mask having a fine pattern is formed by performing lithography on a photoresist. The resist mask having the fine pattern thus formed is generally consumed by etching due to its low plasma resistance. Therefore, the fine pattern of the resist mask is transferred to an inorganic film by plasma etching, as necessary, and the fine pattern transferred to the inorganic film is transferred to a thick organic film having plasma resistance. Accordingly, a mask made of an organic material (hereinafter, may be referred to as an “organic film mask”) is formed.
Next, a porous film is etched by plasma etching in order to transfer the fine pattern of the organic film mask. A SiO2 cap or the like may be formed between the organic film mask and the porous film. In the case of etching the porous film by a dual damascene method, a TiN film having an opening may be formed between the organic film and the porous film in order to form an opening at a desired position of the porous film in a self-alignment manner even when desired positioning accuracy cannot be obtained by performing the lithography on the photoresist. In any of the above cases, the organic film mask used for the transfer operation is removed by ashing using a plasma of a gas containing oxygen or hydrogen.
In removing the organic film mask, radicals are generated by exciting a processing gas for ashing in a processing chamber of a plasma processing apparatus. The radicals may enter pores of the porous film to damage the porous film. For this reason, a porous film needs to be protected from the radicals.
In order to protect the porous film from the radicals, there has been proposed a technique for protecting the porous film from radicals during the plasma etching of the porous film. For example, Reference 1 (Liping Zhang et al., “Damage Free Cryogenic Etching of a Porous Organosilica Ultralow-k Film”, ECS Solid State Lett. 2013 volume 2, issue 2, N5-N7) describes a technology in which the porous film is etched under an extremely low temperature to condense a reaction product in the porous film. In this technology, the reaction product condensed in the porous film restraints the radicals from entering the porous film. In order to condense such a reaction product, a temperature at the time of etching the porous film is set to be lower than or equal to −70° C.
In addition, Reference 2 (Markus H. Heyne et al., “Quantitative characterization of pore stuffing and unstuffing for postporosity plasma protection of low-k materials”, Journal of Vacuum Science & Technology B32, 062202 (2014)) describes a technology in which the porous film is permeated with a polymethylmethacrylate resin (PMMA), and the PMMA restraints the radicals from entering the porous film. In this technology, after the etching of the porous film is finished, the PMMA is removed by a plasma process using a mixed gas of a hydrogen gas and a helium gas, or by a post-treatment such as laser annealing.
In asking for removing the mask in an extremely low temperature environment disclosed in Reference 1, it is impossible to use the plasma processing apparatus which includes a cooling mechanism using a usual cooling medium, and it is necessary to use a plasma processing apparatus which includes a cooling mechanism using, for example, liquid nitrogen or the like. In addition, in order to apply the technique disclosed in Reference 2 to the ashing for removing the mask, a step of allowing the PMMA to permeate the porous film is necessary, and a dedicated processing apparatus is necessary. Further, in the technology described in Reference 2, the porous film may be damaged by the post-treatment for removing the PMMA.
Therefore, an improved technique capable of reducing damage of the porous film is necessary in order to remove the mask, which is made of an organic material and is formed on the porous film.